The human back is a complicated structure including bones, muscles, ligaments, tendons, nerves and other structures. The spinal column consists of interleaved vertebral bodies and intervertebral discs. These joints are capable of motion in several planes including flexion-extension, lateral bending, axial rotation, longitudinal axial distraction-compression, anterior-posterior sagittal translation, and left-right horizontal translation. The spine provides connection points for a complex collection of muscles that are subject to both voluntary and involuntary control.
Muscles provide mechanical stability to the spinal column. Cross sectional images of the spine demonstrate that the total area of the cross sections of the muscles surrounding the spinal column is much larger than the spinal column itself. Additionally, the muscles have much larger lever arms than those of the intervertebral disc and ligaments. The motor control system sends signals down nerves to activate the muscles of the back in concert to maintain spine stability.
The multifidus is the largest and most medial of the lumbar back muscles. It consists of a repeating series of fascicles which stem from the laminae and spinous processes of the vertebrae, and exhibit a substantially similar pattern of attachments caudally. These fascicles are arranged in five overlapping groups such that each of the five lumbar vertebrae gives rise to one of these groups. At each segmental level, a fascicle arises from the base and caudolateral edge of the spinous process, and several fascicles arise, by way of a common tendon, from the caudal tip of the spinous process. Although confluent with one another at their origin, the fascicles in each group diverge caudally to assume separate attachments to the mamillary processes, the iliac crest, and the sacrum. Some of the deep fibers of the fascicles which attach to the mamillary processes attach to the capsules of the facet joints next to the mamillary processes. All the fasicles arriving from the spinous process of a given vertebra are innervated by the medial branch of the dorsal ramus that issues from below that vertebra.
Normally, load transmission in the spinal column is painless, with the muscles acting in concert with the ligaments and bones preventing excessive relative movements of the structures. The neutral zone is the range of intervertebral motion, measured from a neutral position, within which the spinal motion is produced with a minimal internal resistance. Over time, dysfunction of the spinal stabilization system can lead to instability and abnormal movement of the spine, resulting in overloading of structures when the spine moves beyond its neutral zone. High loads can lead to inflammation, disc degeneration, ligament damage, facet joint degeneration, and muscle fatigue, all of which can result in pain.
For patients believed to have back pain due to instability, clinicians first offer a group of therapies that attempts to minimize the abnormal range of motion that leads to the pain. If this group of therapies does not work, then the next group of therapies aims to block the pain produced by the abnormal range of motion.
Common conservative methods of attempting to reduce abnormal motion aim to improve muscle strength and control and include core abdominal exercises, use of a stability ball, and Pilates. If conservative methods of preventing abnormal movement are ineffective, surgical approaches may be used.
Spinal fusion is the standard surgical treatment for chronic back pain. One or more vertebrae are surgically fused together to prevent relative motion. Following fusion, motion is reduced across the vertebral motion segment. Dynamic stabilization implants are intended to reduce abnormal motion and load transmission of a spinal motion segment, without fusion. Total disc replacement and artificial nucleus prostheses also aim to improve spine stability and load transmission while preserving motion.
If pain persists after physical therapy or surgical intervention to prevent the abnormal motion that leads to pain, few options are available for relief.
One option is a technique referred to as “RF rhizotomy”, in which radio frequency (“RF”) energy is used to ablate the medial branch of the dorsal ramus that contains the afferent fibers responsible for transmitting pain signals from the facet joint. There are several devices available for performing this treatment, such as those offered by Baylis Medical Inc. (Montreal, Canada). While this technique can be effective, it provides only short term relief as nerve fibers may regenerate over time, and generally the procedure must be repeated approximately every six months to maintain effective pain control. The electrical parameters for RF ablation of nerves differ amongst various suppliers.
Another option for pain relief is Transcutaneous Electrical Nerve Stimulation (TENS). This technology provides low energy electrical signals delivered via externally applied skin pad electrodes. While the exact mechanism of action is still subject to some controversy, it is generally believed that the electrical energy blocks the signals in the afferent nerve fibers that transmit the pain signals to the brain.
A modification to this approach is to use percutaneous wires connected to electrodes placed nearer to the nerves (PENS or Percutaneous Electrical Nerve Stimulation). A wide variety of PENS or TENS stimulation parameters have been published, including high-frequency (HF; >10 Hz), low-frequency (LF; <10 Hz), variable-frequency (VF) and acupuncture-like (AL), which employs very low-frequency, high-amplitude stimulation. The intensity of the TENS or PENS stimulation (voltage or current) is generally adjusted to a level which achieves analgesia without causing irritation or pain from the stimulation itself. One such PENS device is described in U.S. Pat. No. 6,671,557.
Implantable devices for electrical stimulation of peripheral nerves for control of pain have been described. For example, U.S. Pat. No. 7,324,852 B2 describes an implantable electrical stimulation device with a plurality of electrodes that are implanted subcutaneously and are stimulated in a pre-determined pattern to provide pain relief.
A Spinal Cord Stimulator (SCS) is an implanted electrical stimulation device with one or more electrodes that are placed adjacent or near to the spinal cord, with the goal of blocking the pain signals from being transmitted via the spinal cord to the brain. Although SCS was originally designed and approved for radicular pain (sciatica), the technique is increasingly being used for lower back pain. Spinal cord stimulators may be self-powered (i.e., contain a primary battery or cell) or may include a rechargeable battery (i.e., a secondary battery or cell), as described for example, in U.S. Pat. No. 6,516,227.
The key drawback with all of the previously known electrical stimulation techniques that seek to block the pain signals (TENS, PENS, SCS and RF Ablation of the nerves) is that relief, if obtained, is usually only temporary, and repeated or continuous therapies are needed.
U.S. Patent Application Publication No. US2008/0228241 to Sachs, assigned to the assignee of the present invention, describes an implanted electrical stimulation device that is designed to restore neural drive and rehabilitate the multifidus muscle. Rather than masking pain signals while the patient's spinal stability potentially undergoes further deterioration, the stimulator system described in that application is designed to reduce the propensity for instability of the spinal column, which in turn is expected to reduce persistent or recurrent pain.
While the stimulator system described in the Sachs application seeks to rehabilitate the multifidus and restore neural drive, it does not provide relief of the pain during the application of the therapy. Thus, it is possible that for some patients the effectiveness of the therapy may be hindered by the continuation of pain, which may interfere with restoration of neural drive to the muscle or impede the patient's ability to tolerate the therapy. In addition, it is possible that as the tone of the multifidus muscle improves during use of the stimulator system described in the Sachs application, it may be desirable to reduce the stimulus amplitude, frequency or duration, or stimulation intervals.
In view of the foregoing, it would be desirable to augment the stimulator system described in the Sachs application with additional therapeutic modalities, such as the ability to alleviate pain during and between muscle stimulation. It therefore may be desirable to provide pain blocking stimulation to afferent nerve fibers simultaneously with muscle stimulation pulses, or at other times.
It further may be desirable, depending upon the severity of the pain experienced by a patient and the degree to which it interferes with rehabilitation of the multifidus muscle, to provide pain blocking by selectively ablating afferent nerve fibers in conjunction with the stimulation therapy described in the Sachs application.
It also would be desirable to combine the rehabilitative stimulation therapy described in Sachs with a capability to monitor muscle performance during the stimulation therapy, and to adjust the applied stimulation pulses to account for changes in the muscle tone and neural drive. In addition, it would be desirable to detect the duration, frequency and strength of muscle contractions to further reduce the patient's perception of pain resulting from the muscle stimulation therapy, for example, to avoid spasm.